This invention relates to a method of installing a rivet in a plurality of workpieces, and to the resulting riveted joint, as well as the rivet itself.
One rivet-type fastener commonly used in aircraft constructions includes a shank with a manufactured head on one end and a tail on the other end. In use, the tail end of the shank is inserted through aligned holes of two or more workpieces with the rivet head engaging the outer face of one or the workpieces and with the tail extending beyond the outer face of the other workpiece. The tail is then deformed by means of an axial force, compressing the rivet axially and upsetting the tail material outwardly to form an upset head which is larger in diameter than the hole through the workpieces, so that the two workpieces are fastened together.
In one highly successful rivet, or shear pin, of this general type, the shank which extends through the workpieces is made of a strong material which is high in shear strength while the tail is made of a more ductile material which is easier to deform than the shank. The two metals are typically joined together by friction welding. In one example of this type, the shank is formed of (95 ksi shear) 6AL-4V titanium alloy while the tail is formed of 55Ti 45Cb titanium alloy. Such a fastener is sold by the assignee of the present invention under the trademark Cherry BUCK. U.S. Pat. No. 3,848,389 issued to Gapp, et al. further describes bimetallic shear pins of this general type.
All types of fasteners having a tail to be upset are often installed by squeezing, wherein the ductile tail is compressed until the upset head formed attains a diameter of about 1.3 to 1.5 times the initial shank diameter. When the squeezing force used to form the upset head is released, the column of the rivet shank "springs back" or lengthens a certain distance. Although the material of the workpiece being fastened also springs back, some of the materials in common use do not spring back as much as the rivet shank, with the result that a small gap is created between portions of the upset head and the workpiece after the installation is complete. For example, a solid rivet of 2117T3 aluminum, used for many years in large quantities in the aircraft industry and commonly referred to as an AD rivet, when installed in 7075T6 aluminum material can exhibit a small gap between the upset head and the workpiece. However, some gap is acceptable for most practical uses except those involving high fatigue loads.
In aircraft structures, particularly those involving shear fatigue loading of the fastener, it is desirable that the gap between the upset head and the workpiece be zero. Ideally, the underside of the upset head should exert a compression force against the workpiece after the installation. When such a loading is achieved, the fastener is said to exert a residual tension force against the workpiece after installation. This loading is often referred to as a "preload" in the joint.
While millions of rivets of the type having a ductile tail and a strong shank have been used satisfactorily, their use has been limited in areas involving shear fatigue loading due to the inability to provide a preload. A fastener of this type which provides a preload is usable in a much extended range of applications, particularly those involving shear fatigue and light tension fatigue or a combination of both. Further, the ability to use a bi-metallic fastener in fatigue applications is of greater benefit to the aircraft industry because it is the lightest shear pin type fastener available with a 95 ksi shear rating. In the aircraft industry, weight savings are very valuable. Savings of one pound of weight can be worth $1,000 to the designer in some critical areas of an aircraft design.
Another type of shear pin fastener consists of a bolt with a small head on one end and a small light nut threaded on the other end. The small head and nut are of sufficient strength to develop close to the full shear strength of the fastener when it is installed in two or more workpieces, but not of sufficient strength to develop the full tensile strength of the fastener. In joints involving high axial tension loads, tension type fasteners are used. Typically, these have larger and stronger manufactured heads and larger and stronger matching nuts. These tension type fasteners are, of course, heavier. The size and weight of the squeezed tail of the one piece shear pin herein described is much smaller than the nut and threaded portion of a threaded pin and nut-type shear pin fastener of the same strength. Also, the small height and size of the squeezed tail of the shear pin is advantageous with regard to the installation and positioning of other components in crowded areas within aircraft or other assemblies.
A one-piece fastener is particularly desirable as opposed to a two-piece fastener in that it is easy to feed and install using automatic equipment. Most fasteners of this general type which are capable of providing a significant preload are of the two-piece design. Two-piece designs involve serious feeding problems when automatic or robot installation is attempted. Automated installation of fasteners is advancing rapidly in the aircraft industry because it is cheap and produces much more uniform and satisfactory joints. In the past, preload in a riveted joint has been achieved by using hot rivets which, after being upset, contract upon cooling and produce the desired preload in the joint. This hot rivet approach of course adds other costs and complexities; and while it is still used in large commercial structures, such as bridges, it is not used in aircraft structures.
Accordingly, a substantial need exists for a method of installing a one-piece, cold, upset rivet or shear pin fastener in a manner which can provide a significant axial preload.